1. Field of the Invention
Priority is claimed on Japanese Patent Application No. 2003-315725, filed Sep. 8, 2003, the content of which are incorporated herein by reference.
The present invention relates to a method for starting up a fuel cell stack at subzero temperatures, relates to a system for starting up a fuel cell stack at subzero temperatures, and relates to a method of designing a fuel cell stack.
2. Description of Related Art
Among the fuel cells, there are those in which a solid polymer electrolyte membrane is sandwiched between an anode electrode and a cathode electrode so as to form a membrane electrode assembly. This membrane electrode assembly is further sandwiched between a pair of separators so as to form a single cell (i.e., a fuel cell unit). In this type of fuel cell, typically, a plurality of single cells are stacked and used as a fuel cell stack.
In this fuel cell, a chemical reaction is caused by supplying a fuel gas (e.g., hydrogen gas) to a power generating surface of the anode electrode and by supplying an oxidizing gas (e.g., air that contains oxygen) to a power generating surface of the cathode. The electrons that are generated between these two are then removed to an external circuit and are used as DC electrical energy. As a result of oxidizing gas (e.g., air containing oxygen) being supplied to the cathode electrode, hydrogen ions, electrons, and oxygen react at the cathode electrode and water is created. In this manner, because fuel cells have a minimal effect on the environment, they have attracted attention as driving sources for vehicles.
Moreover, typically, the operating temperature of this type of fuel cell is approximately 70° C. to 80° C., and temperature control is conducted by supplying coolant to coolant flow passages that are provided in the separators such that the fuel cell does not exceed this operating temperature due to the heat that is created when power is generated.
In this type of fuel cell, because the power generating efficiency is deteriorated at low temperatures, startability at low temperatures causes considerable problems. Accordingly, when the fuel cell is used in a vehicle, if an attempt is made to start up the fuel cell when the outside temperature is low, for example, is a subzero temperature, the problem arises in that a considerable time is required before startup is achieved.
As a measure of countering low temperatures, as is disclosed, for example, in Published Japanese Translation No. 2000-512068 of the PCT International Application, the reaction is accelerated by supplying power to the external load of the fuel cell, so that the temperature is raised by self-generated heat and the startability is improved.
If a fuel cell stack is warmed up by its own self-generated heat in this manner, there is a method in which the heat generation is accelerated by supplying a large current to the fuel cell stack in order to shorten the warm-up time.
However, if a shortening of the warm-up time is achieved and the output current is increased, then at the same time as the quantity of generated heat increases, the quantity of water that is generated inside the cells as power is generated also increases. As a result of this generated water freezing inside diffusion electrode layers and catalytic layers, the problem arises in that the reaction gas is unable to reach the solid polymer electrolyte membrane, thereby inviting an abrupt voltage drop and, ultimately, hastening a drop in voltage.
Namely, regardless of how much the output current is increased, if the freezing of the generated water is more rapid than the increase in temperature provided by the self-generated heat, the fuel cell stack ends up becoming unable to generate power due to the water generated inside the cells freezing before the temperature is increased, resulting in the objective not being achieved.
Moreover, regardless of what attempts are made to increase the output current, the maximum current density that can be output in the membrane electrode assemblies that form the fuel cell is decided in accordance with the temperature, and more current than this cannot be supplied.
In addition, if water generated in the diffusion electrode layer and catalytic layer freezes and there is a failure in the startup, it is extremely difficult to once again conduct a startup operation. Generally, when a fuel cell is stopped, a purge is made by supplying gas or the like, so that generated water is not left in the diffusion electrode layer and the like. Accordingly, by supplying reaction gas to the fuel cell stack at the time of an initial startup even at a subzero temperature, it is possible to extract power temporarily from the fuel cell stack. However, once the holes in the diffusion electrode layer and catalytic layer have been blocked by the freezing of the generated water so that the reaction gas is unable to pass therethrough, even if reaction gas is supplied to the fuel cell stack, the reaction gas cannot reach the solid polymer electrolyte membrane and power cannot be obtained from the fuel cell stack. If power cannot be obtained from the fuel cell stack, then it is not possible for the fuel cell stack to be warmed up by self-generated heat. Accordingly, when starting up a fuel cell stack at a subzero temperature, the initial startup operation is extremely important. If there is a failure in the warm-up in the initial startup operation, then, in some cases, the fuel cell stack enters a state in which is it is unable to be restarted.
It is an aim of the present invention to provide a method for starting up a fuel cell stack at a subzero temperature and a system for starting up a fuel cell stack at a subzero temperature that enable warming up to be conducted rapidly before a drop in voltage is generated as a result of the freezing of generated water, and to provide a method for designing a fuel cell stack that is suitable for this subzero temperature startup method and subzero temperature startup system.